Membrane lipids — What characteristic structural features does a typical biological phospholipid contain in its amphipathic architecture?

Introduction:
Phospholipids are the principal building blocks of cellular membranes. Their amphipathic nature drives spontaneous bilayer formation and underlies membrane fluidity, curvature, and protein association. This question tests recognition of the core structural motif of phospholipids.

Given Data / Assumptions:

• Glycerophospholipids contain a glycerol backbone, two fatty acyl chains, and a phosphate-containing headgroup.
• Sphingophospholipids have a sphingosine backbone but retain amphipathic design.
• Amphipathic molecules possess both polar and nonpolar regions.

Concept / Approach:
The hydrophilic (polar) headgroup, often carrying a charge (e.g., choline, ethanolamine, serine, inositol), interfaces with water, while hydrophobic fatty acyl tails cluster away from water. This duality drives bilayer assembly with heads facing the aqueous phases and tails forming the hydrophobic core.

Step-by-Step Solution:

Verification / Alternative check:
Liposome formation in vitro and the classic fluid mosaic model of membranes demonstrate amphipathic self-assembly. Cryo-EM and X-ray scattering confirm bilayer thickness and tail packing consistent with this architecture.

Why Other Options Are Wrong:

• (b) Water-soluble long carbon chains: pure hydrocarbon chains are hydrophobic, not water-soluble.
• (c) Exclusively positive groups: real phospholipids carry polar/charged headgroups plus hydrophobic tails.
• (d) Combination of incorrect statements remains incorrect.
• (e) Purely nonpolar molecules cannot form stable bilayers in water.

Common Pitfalls:
Assuming all headgroups are positively charged; many are zwitterionic or negatively charged, but all contribute to hydrophilicity.

Final Answer:
hydrophilic heads and hydrophobic tails.